Wireless time division multiple access (TDMA) systems are known, such as Group Speciale Mobile (GSM), TETRA and dispatch systems such as iDEN. Such dispatch systems communicate packet data as well as dispatch information such as voice information via shared channels that are shared by a number of wireless units referred to herein as subscribers. With TDMA wireless communication systems that employ subscribers with half duplex transceivers, the subscriber device needs to switch between outbound and inbound traffic because such devices cannot transmit and receive at the same time. The protocol used in such TDMA systems have different slotting protocols for audio information, for example, versus packet data. Typically, a wireless network element will allocate slots for a given subscriber to accommodate a real time voice communication such as a voice dispatch and allocate remaining slots to the extent available, for packet data. Such protocols, as known in the art, also use one or more control slots which are detected by all subscribers. For example, a subscriber may turn off to save battery life during slots that are not assigned to that particular subscriber, but “wake up” to receive the control slots to facilitate further communication or control desired by the system. The control slots may be used, for example, to indicate to a specified subscriber which upcoming remaining slots are to be used as packet data. Such control slots, for example, may be a type of a map identifying for the various channels, which of the TDAM slots in the next packet data frame are allocated for voice versus packet data, for example. This dynamic channel allocation allocates left over slots for data after voice information is taken care of to provide an efficient use of air resources. As also known in the art, a subscriber typically requests a reservation for an inbound channel so that it can transmit to the wireless network element, such as a base radio in an iDEN system. The network element provides the reservations based on a reservation request. Base radios may be operatively coupled to access controller gateways (ACG's) that allocate radio resources for calls and allocate unused slots for allocation as packet channels. The access controller gateway may be coupled to the Internet or any other suitable network via a mobile data gateway (MDG). The access controller gateway and any plurality of base radios typically form a site enhanced base transceiver system that may also include, for example, a dispatch application processor that selects a voice channel for a dispatch communication. In addition, outbound reservations are made by the base radios, for example, to allow outbound communication from the base radio to a subscriber.
In the outbound direction, one channel is typically allocated to serve the subscriber population in a specific cell. Outbound packets are fragmented into a sequence of slot blocks with the first block indicating the destination subscriber. A portion of the outbound channel is reserved for, and ultimately accommodates, the outbound transmission of the message. For battery saving purposes, all other subscribers will either sleep or monitor neighbor cells for the advertised duration of the new outbound reservation with one exception. All subscribers in an active session with fixed network equipment (FNE) must wake up or return to the packet channel to periodically monitor control slots (also referred to as a dynamic channel allocation protocol (DCAP) slots) and additionally to monitor header blocks as outbound message lengths dictate.
On the inbound side, an inbound channel is typically in one of two modes. In the random mode, subscribers can request an inbound reservation for as many as 81 slots, for example. In addition, a reserve mode can be used wherein a single subscriber may reserve the inbound channel and has exclusive use of it for transmitting an inbound message. While the inbound channel is reserved, all subscribers wishing to make an inbound reservation request must refrain from doing so until the inbound mode reverts back to the random access mode for that channel.
A problem arises with wireless subscribers that communicate in a half duplex mode since in some instances, a message collision can occur wherein a subscriber that is the intended target of an outbound message will transmit an inbound reservation request at the same time that the network element begins transmitting the outbound message based on an outbound reservation. For example, a network element may be sending information in an outbound slot but if it receives an inbound reservation request on the slot sent at the same time the outbound slot was sent for the same subscriber, a collision can occur. If a collision occurs, and a subscriber misses a header of a long message, the entire message is lost since the header includes the requisite information indicating which subscriber the message is intended for and the length of the message. A subscriber is typically unable to obtain identification information in the middle of a message. Subscribers typically wake up to listen to header blocks of a message and turn off during the rest of the message blocks if it is determined that the message is not intended for them. In addition, subscribers wake up for the control slot information as well.
Some systems have attempted to overcome collisions by defining certain slots to allow reservation requests. Hence no collisions typically occur since the network element can predict when a reservation may occur for a given subscriber. However, such a solution typically introduces a delay in the inbound message since the subscriber has to wait to send the inbound message until after the designated reservation request slot is available. Delays may also occur in the outbound mode since the network element has to wait to send an outbound message to make sure it is not sending at a time slot that has been allocated for a reservation request.
Other systems attempt to detect collisions and simply resend the message and effectively ignore the collision. This can waste substantial amounts of bandwidth since the transmission is not terminated.
Accordingly, a need exists for a method and apparatus that suitably accommodates and improves performance when a half duplex collision is detected.